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How Can a Fire Risk Approach Be Applied to Develop a Balanced Fire Protection Strategy

By
Man-Cheung Hui
| Fire Protection Engineering

INTRODUCTION
Quite commonly, the first thing the fire
protection engineer may do when engaged in a development project is to
consult the relevant building codes and regulations to seek guidance and
to establish the legal requirements. However, in a scenario where the
development consists of types of occupancies that are not explicitly
defined in the building codes and regulations, how will the fire
protection engineer complete this job?

In another scenario, the fire protection
engineer may face a different type of problem involving various
constraints to the construction of the development, for example, tight
financial budget, site limitations, operational requirements of the
development, and regulatory requirements that may adversely affect the
functionality of the development. Under this circumstance, how can the
fire protection engineer juggle various fire protection requirements?

The article discusses how a fire risk
approach can be employed to assist in the formulation of a fire safety
strategy.

Fire Safety Strategies and Measures
A fire safety strategy can be defined as a
plan on how to use one or a combination of fire protection measures to
achieve predetermined fire safety objectives. There could be one or more
fire safety objectives in a single project. Typical fire safety
objectives include, but are not limited to, the following:

Building occupants' safety.

Property and contents protection.

Business continuity.

Adjacent property protection.

Protection of firefighters.

Averting a catastrophic loss.

Environmental protection.

It is possible that when the fire safety
objectives are changed, the fire safety strategy may need to be modified
accordingly. As a general guideline, the Fire Safety Concepts Tree1 as described in NFPA 550 and shown in Figure 1 facilitates understanding the possible ways to achieve fire safety objectives.

The NFPA Fire Safety Concepts Tree is a
qualitative approach to presenting fire safety in a manner that ensures
all major items of fire prevention, protection, and administration are
examined in a logical fashion. It uses a similar diagram to Fault Tree
Analysis to show relationships of fire prevention and fire damage
control strategies.

It can be seen from Figure 1 that,
fundamentally and philosophically, fire safety can be achieved by not
letting the fire happen (Prevent fire ignition branch) or by dealing
with the fire after it has happened (Manage fire impact branch). The
Prevent fire ignition branch suggests that there could be three ways to
avoid ignition: (a) by controlling fuel (through elimination of fuel or
altering fuel ignitability), (b) by controlling source fuel interactions
(through separation of fuel from heat-energy sources or control of heat
transfer processes), or (c) by controlling heat-energy sources ( through
elimination of the heat sources or controlling the rate of energy
release). Reference can be made to the Fire Protection Handbook2 for a detailed listing of fire-prevention factors.

If the fire prevention measures were 100
percent effective at all times, then nothing else would be needed, and
all fire brigades could be disbanded because there would be no fires. In
reality, no systems or measures are 100 percent effective, and it is
impractical to completely prevent the ignition of fires in a built
environment; therefore, backup measures will be necessary to cater to
scenarios in which ignition is possible.

According to the Fire Safety Concepts
Tree, managing the impact of fires can be achieved by either managing
the "exposed," i.e., occupants, contents, building fabric, operations,
environment, or heritage, depending on the fire safety objectives being
considered, or by managing the fire.

A detailed illustration of the Manage exposed branch can be found in the Fire Protection Handbook2 and in a recent article in this magazine on fire alarm systems and interior finish. 3 In
short, managing the exposed can be achieved by limiting the amount
exposed or by safeguarding the exposed. The former method means that the
number of people or amount of contents in a space is restricted, which
may be impractical. To safeguard the exposed is a much more common
tactic used in building projects, and fire protection measures are often
specified by prescriptive building codes and regulations to achieve
this objective.

There are two primary ways to safeguard exposed: (i) to move
the exposed, i.e., to relocate them away from the hazardous area to a
temporary safe place (e.g., staircases or protected passageways) or to a
safe place (e.g., on the street or an open area), or (ii) to defend
them in place, i.e., to maintain the building space to be tenable for a
sufficient period of time after the start of the fire.

The fire protection measures that may be
employed to safeguard the exposed are summarized as follows:

(a) Potential measures for "move the exposed" strategy;

Fire detection system.

Fire alarm system.

Egress system.

Fire-resistant elements.

Fire suppression system.

Smoke management system.

Fire emergency management system to
protect the escape route, set up emergency control organization, and
implement emergency procedures.

Emergency lighting and exit signs.

Intercommunication system for
communication between occupants and fire wardens or emergency services
personnel.

(b) Potential measures for "defend in place" strategy;

Fire-resistant elements.

Fire suppression system.

Smoke management system.

Emergency lighting.

Intercommunication system for
communication between occupants and fire wardens or emergency services
personnel.

Selection of Fire Safety Strategy and Protection Measures
One of the major tasks for the fire
protection engineer may be to determine which strategy works best for
the building/facility and the occupants therein, and then determine what
fire protection measures should be provided after the strategy has been
selected.

The fire prevention strategy should be considered first, following
the commonly accepted wisdom that "prevention is better than cure."
Research4 shows that fire prevention programs could reduce
the number of fire incidents, but not totally eliminate them, and
refresher courses may need to be held regularly to maintain the same
performance of the programs over the years.

Currently, there is much debate onto
whether the "move the exposed" strategy or the "defend in place"
strategy should be adopted. There is similar deliberation about the
supremacy of active fire protection systems versus passive fire
protection systems. For the latter, it has to be realized that different
fire protection systems work at different phases of fire development;
for example, smoke detection and sprinklers typically actuate in the
early phase of the fire, nonrated building elements or barriers that
have inherent fire resistance may contribute in delaying the spread of
fire and smoke in the growth phase of the fire, and fire-rated barriers
serve to contain the fire in the fully developed phase of the fire.

At the strategy level, qualitatively, it
may be quite straightforward to judge that moving the exposed is not
appropriate for certain occupancies, such as the intensive care units in
hospitals, nursing homes, detention and correction facilities, and the
like. However, there are some occupancies and some circumstances where
either the "move the exposed" strategy or the "defend in place" strategy
may be desirable, and there may be a combination of fire safety
measures that can be employed in each strategy. In this case, an
assessment more robust than a qualitative judgment may need to be
employed to assist the decision making process. Quantitative fire risk
assessment is one of the assessment methods that are suitable for
evaluating the options within each strategy.

Quantitative Fire Risk Assessment
There are many definitions and views of
risk, depending on which part of society or which application is
considered. There is even a positive side to risk in the business
context, because innovation and development involve risk. In the fire
safety context, fire risk is typically associated with fire hazard, and
often mistakenly solely associated with fire hazard. The following
example shows how risk and hazard are not necessarily directly
proportional.

Imagine there is a five-kilogram hammer resting loosely on the
top rung of a two-meter-high ladder. If the ladder-hammer combination is
locked in a storeroom, the hazard has not changed but the risk of
someone getting injured is significantly reduced. If the same
ladder-hammer combination is placed on a busy sidewalk, the hazard
remains the same but the risk of someone getting hit by the hammer is
considerably increased.

Where it is required to determine whether the "move
the exposed" strategy or the "defend in place" strategy would be more
appropriate for a particular occupancy, one could utilize the
aforementioned "fire risk" concept and conduct a fire risk assessment to
provide guidance.

Fire risk assessment usually forms part of a fire risk
management program; however, the assessment exercise is often carried
out separately without referring to other parts of the management
program, which may run the risk of missing the big picture. A typical
fire risk management process is illustrated in Figure 2.

It can be seen in Figure 2 that the risk
assessment exercise consists of three major parts conducted in sequence,
i.e., fire hazard identification, then fire risk analysis, and finally
fire risk evaluation. A full description of the hazard identification,
risk analysis, and risk evaluation process is beyond the scope of this
paper; instead, a simplified version is presented here for practical
purposes.

Fire hazard identification can be carried out in several ways,
e.g., by using a checklist or a HAZOP study. The hazard identification
could:

Identify potential fire ignition sources;

Identify fire load quantity and arrangement that may lead to high-severity fires; and

The personnel who are assigned the task
to identify fire hazards must at least have general knowledge of
combustion, fire safety, and the characteristics of various fire
protection systems, and be very familiar with the operational aspects of
the built environment.

Fire risk analysis consists of determining the
likelihood and consequence of each considered fire scenario. A fire
scenario, for the purpose of quantitative fire risk analyses, can be
considered as a timed sequence of events after an ignition. The
development of these events is dependent on fuel quantity and
arrangement; characteristics of the built environment, such as the
location and status of fire/smoke barriers and that of openings, such as
doors and windows, through which air and smoke could pass; and the
performance of various fire protection measures. A simplified example
fire scenario is illustrated in Figure 3 in the form of a Timed Event
Tree with four branches (a) to (d), and t representing time.

Since there could be an infinite number
of fire scenarios to be considered, and the resources to analyze fire
scenarios are finite, it is necessary to structure the fire scenarios
into a manageable number of scenario clusters for evaluation. A properly
established scenario structure consists of a group of scenario
clusters, each with its own representative fire scenario; the scenario
clusters are non-overlapping and collectively include all relevant
scenarios. The likelihood (i.e., frequency of occurrence) for a scenario
cluster is the sum of the frequencies of all scenarios contained in the
cluster, whereas the consequence for a scenario cluster is estimated
from the representative scenario of the corresponding scenario cluster.

To estimate the frequencies of occurrence
of fire scenarios, the probabilities of the initial ignition, fire
spread, and the probabilities of failure of various fire protection
systems have to be estimated. The frequencies of initial ignition and
probabilities of fire spread are commonly derived from past fire
incident statistics, if such data exist. However, if statistical data
are not available or the quality of the data does not allow meaningful
interpretation, it may be possible to specify the initial ignition
conditions and then employ fire engineering calculation techniques to
predict subsequent development of the fire.

The probabilities of failure of various
fire protection systems ( also known as the reliability of these
systems) could be estimated from past failure data or, more commonly,
derived from fault tree analyses. In a fault tree, a logic tree of 'AND'
and 'OR' gates portrays the combinations of conditions that can lead to
failure of the studied fire protection system, which is defined as the
top event of the tree. For more complex systems that are provided with
standby components or redundancies, determination of their reliabilities
may need to employ mathematical techniques in reliability theories. The
reliability models of three common types of fire protection systems
from O'Connor6 are summarized as follows:

Basic series model The reliability, R,
which has a numerical value from 0 (totally unreliable) to 100
(perfectly reliable), of a fire protection system that has a series of n, statistically independent components (the reliability of the ithcomponent is denoted as Ri), can be described by a series reliability model as follows:

An example of this fire protection system
is a smoke management system with a smoke-extraction fan driven by a
power source that is switched on by a smoke-detection device through a
fire alarm panel. In this system, the components can be considered as
wired in series in terms of signal or power transmission, and failure of
any component will lead to failure of the smoke management system.

Active redundancy model The reliability, R, of the simplest redundant system which consists of two statistically independent components, with reliabilities Ri and R2,
where satisfactory operation occurs if either one or both parts
function, can be described by an active redundancy model as follows:

An example of this fire protection system
is a fire detection system with a number of fire detectors protecting
the same enclosure, where actuation of any one detector or a combination
of detectors will raise a fire alarm.

The general expression for an active parallel redundancy system with n, statistically independent components, is:

Standby redundancy model
The standby redundancy system refers to a
system where one unit does not operate continuously but is only
switched on when the primary unit fails. An example would be a fire pump
system that consists of a duty pump and a standby pump, where the
standby pump will be switched on only after the duty pump has failed.
Another example of a standby redundancy system is a series of fire door
assemblies that are used to subdivide a long corridor. The reliability, R,
of this system with two statistically independent units, assuming both
units have equal constant failure rates λ (with units of number of
failures per unit time) and there are no dormant failures (dormant
failure refers to failure of the component/system in non-operating
conditions) to the sensing and switching systems, can be described as:

where t is the time of operation.

The frequency of occurrence of each fire scenario (i.e., each branch of
the event tree) is the product of the frequency of ignition,
probability of fire spread, and the reliability of the fire protection
systems provided. Alternatively, the frequency of occurrence (denoted as
λ, with units of number of occurrences per unit time) can be converted
into a probability of occurrence (denoted as p) if the design life of the building or the period in which the fire risk assessment is applicable (denoted as T) is known, by the following relationship: 7

Quantification of consequences of each
fire scenario is generally carried out by fire protection engineering
calculations. It is in this step where the performance of fire
protection systems employed in the fire safety strategy has to be
evaluated. It was proposed that the performance of fire protection
systems can be conceptually described by two parameters: efficacy and
reliability, where efficacy is defined as the degree to which a system
achieves an objective given that it operates.8

Once the frequency or probability of
occurrence of each fire scenario and the consequence of the
corresponding fire scenario are estimated, the risk of such fire
scenario can be calculated through the following procedures.

Fire risk can be generally defined as a
probability distribution function over the space of all possible fire
scenarios, together with one or more severity or consequence functions,
also defined over that space.9 Mathematically, the risk
associated with a particular fire scenario originated from a particular
fire ignition source at a particular location can be estimated from the
following relationship.10

where f is the frequency of the particular fire scenario (units of time -1) and C is
the consequence of the particular fire scenario (in units of number of
fatalities, number of injuries, or unit of dollar loss, etc.).

where fiis the frequency of the ithfire scenario cluster (which is the sum of the frequencies of all fire scenarios contained in the cluster), Ciis the consequence of the ithfire scenario cluster, and N is the total number of fire scenario clusters considered.

The last step of the fire risk assessment
is evaluation of the fire risk. In this step, the total fire risk
estimated by the method described above for different distinct fire
protection strategies can be compared for making risk-informed
decisions. The fire risk assessment method can also be used to evaluate
the total fire risk of fire safety strategies that place emphasis on
different fire protection systems, for instance, one that relies heavily
on active fire protection systems versus the other that relies heavily
on passive fire protection systems. These computed fire risk levels also
can be combined with a cost model for conducting a cost-benefit
analysis.

Addressing Uncertainties in Quantitative Fire Risk Assessments
In conducting a fire risk analysis, one
would need to realize that many of the parameters used in the risk
estimation may have significant uncertainties. These may include errors
incurred during the simplification process of the problem, the
statistics on which the frequencies of occurrence or probabilities are
derived, the reliabilities of fire protection systems, and the
calculation methods and models used. It should be recognized that there
are other types of uncertainties, such as perception uncertainties,
public acceptance uncertainties, behavior uncertainties, and similar
nonengineering uncertainties; however, addressing these uncertainties is
beyond the scope of this paper.

A detailed discussion on the topic of
uncertainty in fire safety engineering calculations has been given by
Notarianni.11

Gamache, S., Porth, D., and Diment, E.,
"The Development of an Education Program Effective in Reducing the Fire
Deaths of Preschool Children," Proceedings of the 2nd International Symposium on Human Behavior in Fire, Interscience Communicaions, London. 2001.

* Reprinted with permission from NFPA 550-2002, Fire Safety Concepts Tree,
Copyright 2002, National Fire Protection Association, Quincy, MA 02269.
This reprinted material is not the complete and official position of
the NFPA on the referenced subject, which is represented only by the
standard in its entirety.

About SFPE

SFPE is a global organization representing those practicing in the fields of fire protection engineering and fire safety engineering. SFPE’s mission is to define, develop, and advance the use of engineering best practices; expand the scientific and technical knowledge base; and educate the global fire safety community, in order to reduce fire risk. SFPE members include fire protection engineers, fire safety engineers, fire engineers, and allied professionals, all of whom are working towards the common goal of engineering a fire safe world.